Sam
Congrats to your DS on the offer.
The school is very sporty, gets good results and seems to a desirable destination for many 11+ parents on the London circuit (there are ample threads on MN discussing admissions/waitlists/tutors etc).
But it is primarily a Day school. Boarders I think make up fewer than 10% of the student population. Its roughly 1000+ students, of which only 80 are boarders. Spread across 5 age groups, that's fewer than 16 boarding students in your son's year. Note too that some of the boarders are weekly boarders also, i.e. they won't be around on the weekends unless they have sports/club/music commitments. My point here is, if your DS is the easy going or the 'likes-to-be-left-alone' sort then he should be fine. However, if he likes the company of others, or has specific types that he mingles best with then he may find his weekends a little lonely.
That said, the boarding house is new (yes I visited), the furnishings are quite good and the matron and the staff are friendly. Shared rooms in the lower years but there was ample workspace for the boys in each room. The food, however, I heard contrasting comments, some liked it but others said they couldn't wait to go home at weekends to have home cooking instead.
As your son isn't due to start until Sept 2018, I would urge you to visit, if you can, before making your choice.
Are you looking at other schools as well?
If you have further questions feel free to pm me.